I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.

For am I now seeking the approval of man, or of God? Or am I trying to please man? If I were still trying to please man, I would not be a servant of Christ.—Galatians 1:6-10 (ESV)

The bolded part above is odd. Leaving out the reference to "an angel", it reads: "if we should preach to you a gospel contrary to the one we preached to you, let him be accursed." Does Paul expect to change (or rather lose) his mind or is he expecting someone to forge a letter from him? The context of the letter seems to assert authority for himself, so the idea that he and his fellows might preach a contrary gospel seems to contradict his theme.

Context may help inform this. With the "Judaizers" coming in and effectively undoing his earlier ministry, Paul writes to reassert the primacy of the gospel in the Christian life. I don't think that he is arguing for his own authority, I think that he is arguing for that supremacy and that the gospel should be the object of faith and hope and trust - not the person communicating.
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swasheckMar 16 '12 at 18:52

(rough translation:)2 It can be taken as a hyperbolic word: not because either an apostle or an angel preached in another way than once they had said. But even if it could happen, that the apostles and the angels change themselves, one should not desert that which was once accepted.

… as wisdom

This hyperbolic word was thought to be a sign of Paul's wisdom. In his commentary "In epistulam ad Galatas commentarius" Chrysostomus states (1,7):3

See the Apostle’s wisdom; to obviate the objection that he was prompted by vainglory to applaud his own doctrine, he includes himself also in his anathema; …

This wisdom's aim is to avoid any appearance of thirst for glory. Theodoret argues similarly.4

With the tradition in line Martin Luther argues in his 2nd lecture on Galatians (WA 40, 117, 1f., 1531):

Thanks for digging these up and for the more than sufficient translations. I don't read Latin, but your English translation sounds on the mark. (Corrections: "apostel" => "apostle", "vehement" => "vehemently".) Would "one should not desert that which was once accepted" be an alternative to "it however may not withdraw from that, which once was accepted."?
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Jon Ericson♦Mar 19 '12 at 16:51

Paul is at pains in Galatians to show that his message comes directly from Jesus Christ. He insists in 1:1 that he is an apostle (one sent) not from/by men, but by Jesus. The narrative he gives in 1:11-24 is intended to show that it was Jesus who commissioned him, not the apostles in Jerusalem. In 2:6, When he finally goes to meet with the "pillars" in Jerusalem he downplays their importance and insists that "they added nothing to my message." And then in 2:11f, after these pillars have been shown to agree with his gospel, he tells how he had to bring Peter back in line with the truth of it, showing that the truth of the gospel stands even over the apostles in Jerusalem.

Paul is seemingly self-conscious of the authority of his message; but he does not place this authority in himself but in that the words he speaks are the words of God. He preaches as one sent by and on behalf of Jesus Christ. He writes elsewhere to the Thessalonians:

And we also thank God continually because, when you received the word of God, which you heard from us, you accepted it not as a human word, but as it actually is, the word of God, which is indeed at work in you who believe. (1 Thes. 2:13 NIV)

If Paul were to come proclaiming a different gospel - one in contradiction to the message he received from Jesus - it would obviously not be from the Lord. That said, it seems best not to read too much into the statement you have bolded. Paul probably doesn't anticipate that he himself will preach to them a different gospel anymore than he thinks an angel will come to them from heaven.

Instead, he seems to be emphasizing the authority of his message from God over the authority of men who maybe claim to represent James and the other "pillars" in Jerusalem. He wants to emphasize that no matter who is preaching, their message needs to be in line with that of Jesus Christ.